1. Field of the Invention
The present invention relates to X-ray inspection techniques for electrical circuit boards and similar structures. More specifically, the present invention relates to a method for determining the degree of interconnection of solder joints for printed circuit boards, hybrid boards, padgrid arrays and semiconductor packaging.
2. Description of Related Art
In electronics, components are typically mounted upon or inserted into a circuit board. The electrical contact between a circuit board and the components is assured by soldering of the component into permanent position. Thereafter, the electrical integrity of the circuit board depends upon the mechanical integrity of the soldering performed during the circuit board assembly. Soldering processes are well-known and may be reasonably controlled to correct solder-related deficiencies. However, soldering processes do not always work perfectly with deficiencies such as solder skips, bridges, lack of wetting, insufficient amounts of solder, blow-holes and pinholes which can occur as a result of variations in materials in the solder process. Defects, such as those just mentioned, occur sufficiently often such that it is mandatory to inspect solder connections to reduce solder connection related failures.
Early inspections were strictly visual, with the external appearance of the solder connection being used to infer internal structural integrity. However, visual inspection could not verify the uniformity of the solder within the connection, and could not detect defects that are hidden below components mounted onto a flat surface. Solder uniformity has a critical influence on the strength and durability of the solder connection. Solder connection strength and uniformity are particularly important in the connection of surface mount devices where the devices are held entirely by the solder connection. It is well-known in the surface mount device art that solder connections are more susceptible to thermal and mechanical stress related failures than pin mounted devices. In surface mounted devices, visually inspected structurally marginal connections, due to solder non-uniformity, may still provide electrical connection without the defect being discovered in stress testing. As a result, the marginal connection or hidden defect is a likely candidate for a longterm failure while in use under normal mechanical and thermal stress. With an increasing number of surface mount components being used, visual inspections have proven to be too unreliable in detecting structural deficiencies in the solder connections.
Visual inspection systems for solder quality are available to detect defects such as missing components, cracked or incomplete solder joints, misaligned components, and missing or excess solder. In many applications, however, defects are hidden from visual inspections systems. An example of such a defect is in the case of solder porosity and voids in solder connections beneath surface mount devices. While defects may not be masked by visual barriers, increasing circuit density may result in defects which are not readily apparent to the human eye at production line inspection rates. Even with machine visual inspection systems, inspection deficiency still exists. For example, machine visual inspection system would be unavailable for inspecting defects such as unwanted solder balls under a pad-grid array.
The use of X-ray inspection techniques enables the inspection of visually hidden defects. The metallic alloys used in solder are partially opaque to X-rays as compared to the translucence to X-rays of the ceramics, epoxies, silicon or copper materials used in circuit board assemblies. In addition, the ceramics, epoxies, silicon or copper materials have different degrees of translucence so as to permit the distinction between these materials. As a result, small defects in the solder interconnection are readily identified.
X-ray inspection images are effectively three dimensional, i.e. length, width and thickness, with length and width (or size) being represented by object contrast from surrounding areas with thickness being represented by the shades of grey or black. With data corresponding to the size and thickness of the solder connection, a determination can be made as to the quality of the solder connection. An automated system capable of such X-ray inspection is disclosed in U.S. Pat. No. 4,809,308, where solder quality is digitally quantified to provide the ability to set accept/reject criteria.
The X-ray image of a solder joint will most likely appear as a circle which is darker at its center and fades to lighter shades of gray towards the perimeter, the darker center indicating the point of greatest thickness. An applied solder ball which has not been sufficiently reflowed to provide good interconnection will also appear as an X-ray image of a circle which is dark at its center, fading towards lighter shades of gray at the perimeter, with the only possible distinguishing feature being a slight change in the area of the circle. Inadequacies exist, therefore, even in present-art X-ray inspection techniques for determining all types of potential defects. In addition, double-sided circuit boards present a special difficulty for X-ray inspection techniques when a solder joint on one side of the board is directly opposite a solder joint on the other side of the board. The X-ray image in this situation will be of two solder joints superimposed on one another, making it difficult to determine the location of any defects in one or both of the joints.
The technique of X-ray inspection is also applicable to hybrid assemblies and semiconductor packaging, especially flip-chips. In flip-chip packaging rather than using bond wires, solder is applied directly to the pads of the chip which is flipped over to be attached directly to the package leads, making visual inspection impossible.
It would be desirable to have a reliable method of determining whether interconnection has been achieved which is compatible with X-ray inspection techniques and which does not require a substantial change in the board manufacturing process.